Interstellar Marines by Zero Point Software, seems like a fairly nice game for the STEAM Early Access price of under $20. It has a single-player mode, but more importantly a four-player coop mode is already present. The shining uniqueness about the game is that it is 90% community funded and 95% community driven... it also it has space sharks?

Buy it at the STEAM Store: http://store.steampowered.com/app/236370/Buy it at the STEAM Store: http://store.steampowered.com/app/236370/

Over time the developers are releasing small pieces of the game that focus on specific systems - shooting, movement, settings, actual content, etc. Zero Point is made up of two developers who are independant of the big publishing companies and mainstream support. So updates and added content is a very, very, very slow process. This could be a potential problem for some gamers and a deal-breaking-con for others.

Zero Point have put out a video showcasing new footage of the game and bringing you up to speed on its development. Right now you can sign up for a free account at the game's official website and play any of the pieces right in your web browser!